In recent years there has been a marked increase in the sophistication of downhole tools, and in particular, downhole tools deployed in the bottom hole assembly (BHA) of a drill string. A typical BHA commonly includes, for example, one or more logging while drilling (LWD) and/or measurement while drilling (MWD) tools. Such tools are known to include various electronic sensors such as gamma ray sensors, neutron sensors, resistivity sensors, formation pressure and temperature sensors, ultrasonic sensors, audio-frequency acoustic sensors, magnetic sensors, acceleration sensors, and the like, as well as electronic memory to store the various sensor measurements. Moreover, a typical BHA further includes other tools, such as a telemetry tool, a formation sampling tool, and/or a rotary steerable tool, which include electronic controllers disposed to control, monitor, and record various tool functions during drilling.
It is well known in the art that severe dynamic conditions are often encountered during drilling. Commonly encountered dynamic conditions include, for example, bit bounce, lateral shock and vibration, and stick/slip. Bit bounce includes axial vibration of the drill string, often resulting in temporary lift off of the drill bit from the formation (“bouncing” of the drill bit off the bottom of the borehole). Lateral shocks and vibrations are those which are transverse to the axis of the drill string and are often due to impact of the BHA with the borehole wall. Stick/slip refers to a torsional vibration induced by friction between drill string components and the borehole wall. Stick/slip causes rapid rotational acceleration and deceleration of the drill string and is known to produce instantaneous drill string rotation speeds many times that of the nominal rotation speed of the table. Bit bounce, lateral shock and vibration, and stick/slip are commonly recognized as leading causes of electronic failures in downhole tools. These electronic failures often result in costly trips (tripping the drill string in and out of the borehole) to repair or replace damaged tools and/or tool components.
Due in part to the above described dynamic conditions, the use of integrated circuit boards in downhole tools (e.g., in electronic sensors and controllers) poses no small challenge. Typically, downhole tools must be designed to withstand shock levels in the range of 1000 G on each axis and vibration levels of 50 G root mean square. Moreover, integrated circuit boards and circuit board components must typically be able to withstand temperatures ranging up to about 200 degrees C.
Potting and/or encapsulating electronic assemblies to protect them from vibration, shock, and/or thermal exposure is well known. For example, U.S. Pat. No. 4,891,734 to More (hereafter referred to as the More patent), discloses encapsulating an entire electronic assembly (with the exception of a connector) in an elastomeric material. The elastomeric material is molded about the circuit board and shaped to fit a confining enclosure, such as a tubular shroud and/or a strongback.
One drawback with encapsulation processes is that it is difficult to access and repair an electronic assembly once it has been encapsulated. While the encapsulating material can be removed, or partially removed, from the electronic assembly, its removal is time consuming and difficult. Removal of the encapsulating layer or layers also often causes further damage to the electronic assembly (e.g., broken leads and/or solder joints). Another drawback of encapsulation processes is that during encapsulation conformal coating and/or elastomeric encapsulation materials commonly migrate underneath electronic components mounted on the circuit board. Expansion and contraction of these materials with changing temperatures (e.g., during use of a downhole tool in a borehole) is known to fatigue, or even fracture, solder joints, thereby causing failure of the electronic assembly.
While various methods have been employed to overcome the above described problems associated with encapsulation, these methods are not without their own drawbacks. For example, in one known process, a silicon-based caulking material is applied to the component leads to prevent the encapsulating materials from migrating under the components. In another known processes, a tape may be applied to the component leads. While these approaches can be somewhat effective at preventing ingress of the encapsulating materials, they tend to be time consuming and therefore expensive. Both approaches are also known to be susceptible to the build up of static charges that can cause reliability problems and even failure of sensitive integrated circuits. For example, installation and removal of tape from the component leads is known to sometimes cause large and damaging static charges. Thermal expansion and contraction of the caulk is also known to sometimes result in a build up of static charge (as are downhole tool vibrations). The caulking material also tends to further exacerbate difficulties in accessing and repairing an encapsulated assembly.
Gross, in U.S. Patent Publications 2005/0093201 and 2006/0043635 (referred to herein as the Gross publications), discloses an alternative approach in which an electronic circuit board is enclosed in a two-piece molded pre-form. While this approach might be expected to overcome the above described difficulties associated with encapsulation, it is not without its own difficulties. One particular difficulty is that printed electronic assemblies (including the printed circuit boards and electronic components/integrated circuits soldered thereto) do not typically have tight size or placement tolerances. This difficulty may be exacerbated by intermediate fabrication steps such as the aforementioned taping and/or caulking of the component leads. As such, there is difficulty in achieving a consistent snug fit of the pre-forms about the electronic assemblies. This tends to reduce the effectiveness of the vibration and shock isolation afforded by the molded perform since the electronic assembly can vibrate in the molded perform if it is not held snugly therein. The result can be more frequent failure of electronic components downhole. Molded pre-forms also frequently need to be resized (trimmed) in order to fit over the circuit board. Such resizing is time consuming and results in an unacceptably high degree of variability in the “snugness” of the fit.
Therefore, there is a need in the art for an electronic assembly having improved protection from vibration, shock, and thermal exposure, such as experienced, for example, in downhole drilling applications.